Deposition of acetonitrile to the Atlantic Ocean off Namibia and Angola and its implications for the atmospheric budget of acetonitrile

Abstract

We performed airborne measurements of acetonitrile, carbon monoxide, and meteorological parameters in the marine boundary layer and the free troposphere over the South Atlantic Ocean off the Namibian and Angolan coast. The acetonitrile molar mixing ratio in the free troposphere was enhanced due to biomass burning emissions from the continent. In sharp contrast, the marine boundary layer was strongly depleted in acetonitrile with mixing ratios markedly below typical background conditions. This points to significant uptake of acetonitrile by the Atlantic Ocean near Namibia and Angola. We estimate the dry deposition velocity of acetonitrile to be of the order of 0.4 cm s-1. Based on these results and other acetonitrile measurements, we examine the global budget of acetonitrile. This budget is not balanced when the known source and sink strengths are applied, suggesting a missing source of acetonitrile of ∼0.6 Tg a-1 (equivalent to 0.2 Tg a-1 nitrogen).